Recently, wheel speed detecting apparatus has been proposed that includes a magnetic encoder with magnetic characteristics changing along the circumference of the encoder. A wheel speed sensor controls the anti-lock brake system (ABS) by detecting the wheel rotation speed in accordance with the rotation of the encoder. The wheel speed sensor is built in a bearing in order to eliminate complications of air gap adjustment of the encoder and to make the size of the speed detecting apparatus compact.
An example of such a wheel bearing apparatus incorporating the wheel speed detecting apparatus is shown in FIG. 9. This wheel bearing apparatus, incorporating the wheel speed detecting apparatus, is adapted to be mounted on a knuckle of a vehicle (not shown). It includes an outer member 51 and an inner member 52 inserted into the outer member 51, via double row balls 53, 53. The inner member 52 includes a wheel hub 55 and an inner ring 56 fit onto the wheel hub 55.
The outer member 51 is integrally formed with a body mounting flange 51b on its outer circumference. The body mounting flange 51b is to be mounted on a knuckle of a vehicle. The outer member inner circumference includes double row outer raceway surfaces 51a, 51a. The inner member 52 includes double row inner raceway surfaces 55a, 56a opposing the double row outer raceway surfaces 51a, 51a. One inner raceway surface 55a of the double row inner raceway surfaces 55a, 56a is formed on the outer circumference of the wheel hub 55. The other inner raceway surface 56a is formed on the outer circumference of the inner ring 56. The inner ring 56 is press-fit onto a shaft-like cylindrical portion 55b. The cylindrical portion 55b axially extends from the inner raceway surface 55a of the wheel hub 55. Double row balls 53, 53 are contained in a space formed between the outer raceway surfaces 51a, 51a and the inner raceway surfaces 55a, 56a. The balls 53, 53 are rollably held by cages 57, 57.
The wheel hub 55 is integrally formed with a wheel mounting flange 54 on its outer circumference. The wheel mounting flange 54 mounts a wheel (not shown). The inner ring 56 is axially secured by a caulked portion 58. The caulked portion 58 is formed by plastically deforming the end of the cylindrical portion 55b radially outward. A seal 59 and a cover 60 are mounted on ends of the outer member 51. The seal 59 prevents leakage of lubricating grease sealed within the bearing space and entry of rain water or dust into the bearing space from the outside.
An encoder 61 is press-fit on the inner ring 56. The encoder includes a cylindrical mounting portion 61a and an encode body 61b. The mounting portion 61a has a generally annular configuration formed by plastically working a magnetic sheet such as soft steel sheet etc. It is adapted to be press-fit onto the inner ring 56. The encoder body 61b is adapted to be mounted on a flanged portion of one end of the mounting portion 61a. The encoder body 61b is formed by arranging a permanent magnet, such as a rubber magnet, in an annular fashion. The magnet is magnetized so that the magnetic poles N and S are alternately arranged in a circumferential direction to generate magnetic field variation due to rotation of the encoder body 61b. 
The cover 60 is formed with a bottomed cylindrical configuration by injection molding of synthetic resin. A cylindrical portion 60a is fit in an opened end of inner-side of the outer member 51. The opened end of the outer member 51 is closed by a lid portion 60b. As shown in FIG. 10, a cylindrical sensor mounting portion 62 is formed on the lid portion 60b. An insertion portion 64a of a sensor 64 is inserted into a sensor mounting bore 63. The bore 63 is formed in the sensor mounting portion 62. A partition wall 66, with a thickness of 0.3 to 0.5 mm, is formed on the bottom of the sensor mounting bore 63 to separate the sensor mounting bore 63 and an inside space 65 of the bearing. The partition wall 66 is injection molded integrally with the cover 60. Since its material is a non-magnetic synthetic resin, the partition wall 66 does not cause any influence to the detection accuracy.
An outer shell of the sensor 64 is formed from synthetic resin. The sensor 64 is mounted on the cover by inserting the sensor insertion portion 64a into the sensor mounting bore 63. The sensor insertion portion 64a opposes part of the encoder body 61b, via the partition wall 66 of the sensor mounting bore 63. A detection portion (not shown), to detect magnetic variation generated by rotation of the encoder 61, is contained within the sensor insertion portion 64a. It is positioned near the partition wall 66 opposing the encoder body 61b. 
In the prior art wheel bearing apparatus, incorporating the wheel speed detecting apparatus, the partition wall 66, separating the sensor mounting bore 63 and the inside space 65 of the bearing, is provided. The sensor mounting bore 63 does not pass through the cover 60. Thus, less passage of foreign matters is achieved. Also, whole sealability of the apparatus is superior to previous apparatus. In addition, since the partition wall 66 and the cover 60 are integrally molded structure, assembling work of the cover 60 is simplified. See, JP 2004-354066 A.
As described above, the prior art wheel bearing apparatus, incorporating the wheel speed detecting apparatus, includes the partition wall 66 to separate the sensor mounting bore 63 and the inside space 65 of the bearing. Thus, less passage or entry of foreign matters occurs. Additionally, whole sealability of the apparatus is superior to previous apparatus. However, it is known that the detecting accuracy is significantly lowered when the air gap between the encoder 61 and the sensor 64 exceeds 2 mm. Thus, it is necessary to reduce the thickness of the partition wall 66. However, it is believed that the partition wall 66 would be damaged if any foreign matter is pinched in the detecting portion. Thus, sealability would be impaired.
On the contrary, it is also a problem that once foreign matter has entered into the sensor mounting bore 63, it cannot easily be discharged since the sensor mounting bore 63 does not pass through the cover 60.